It is the first time I have met him.中the first time引导的是表语从句吗？

不是表语从句，而是the first time为It is的表语，I have met him 做time的定语。这里的time后面省略了连接词when（或that）。故该句子缩写后为：It is the first time.
the first time只能引导时间状语从句，如：
The first time she saw him, she fell in love with him.
注：区别于for the first time。本句可改写为It is for the first time that I have met him.该句可还原为：I have met him for the first time.可以看出去掉It is...that后，句子完整。故改写后的句子是强调句，而且这里的that不可省略。

不是表语从句，是定语从句。

是的咯
只不过看不看表语从句不是看the first time
而是看前面的动词，如果是系动词的话，那后面跟表语，表语后跟的是修饰表语的定语